Activity 2.6 NCERT Class 10 Science, Chapter 2 Acids, Bases, and Salts.
This activity asks us to add a few drops of phenolphthalein into dilute sodium hydroxide and see what happens when we add dilute acid.
Observation: Adding phenolpthalein into dilute sodium hydroxide turns the solution pink. Now, when we add dilute acid in it, a time comes when solution become colurless again.
Phenolphthalein is a colourless reagent. It gives the pink to the solution at basic PH while at acidic PH the solution remain colour less. So, It is used in an acid-base titration to see the endpoint. In the above experiment ist solution is basic so we see pink colour. Subsequent addition of dilue acid neturalises the solution and at the end makes the solution acidic. As a result, the pink colour disappears.
Note:If we add dilute base after adding acid then situation will reverse and the solution gets own previous pink colour.
Basic solution + phenolpthalein ⇒ Pink colour
Acidic solution + phenolpthalein ⇒ Colourless
Inference/conclusion: A change in colour of PH indicator can indicate the completion of a reaction.